Bathroom Renovation Mistakes That Cost Thousands to Fix
We've seen every bathroom renovation mistake in the book. Here are the most expensive ones and how to avoid them.
After hundreds of bathroom renovations across London, we've seen the same costly mistakes repeated. Many of these happen during the planning stage and become apparent only after the bathroom is finished — when fixing them means starting again. Here are the most expensive errors and how to avoid them.
1. Poor Waterproofing
Cost to fix: £3,000 – £8,000 (full retile and re-tank)
Inadequate waterproofing behind tiles is the single most expensive mistake. Water penetrates grout lines, reaches the substrate, and causes damp, mould, and structural damage. It's invisible until the damage is severe — often appearing as damp patches on the ceiling below or mould growth at tile edges.
How to avoid: Insist on proper tanking in all wet areas. Liquid-applied tanking membrane (BAL, Mapei, or similar) should be applied to walls and floor in the shower zone, with a minimum of two coats and tape over joints and corners.
2. Ordering Wrong Tile Quantity
Cost to fix: £500 – £2,000 (replacement tiles from different batch + labour)
Ordering too few tiles means the project stops while you wait for more — and the replacement batch may have slight colour variation from the original. Ordering too many is far less costly, as unused boxes can usually be returned.
How to avoid: Calculate the exact area, add 10-15% for standard layouts and 15-20% for diagonal patterns or large-format tiles. Always order from the same batch.
3. Ignoring Ventilation
Cost to fix: £300 – £1,500 (fan installation after tiling requires additional work)
A bathroom without adequate ventilation will develop mould on grout, silicone, and painted surfaces within months. Mould damages health and degrades finishes prematurely.
How to avoid: Install a quality humidity-sensing extractor fan as part of every renovation. Budget £150-300 for a good fan (Envirovent Silent, Vent-Axia Lo-Carbon) — it's the cheapest insurance against mould.
4. Not Checking Water Pressure First
Cost to fix: £500 – £3,000 (system upgrade or pump installation after bathroom is finished)
Choosing a large overhead rain shower for a gravity-fed system with 0.2 bar pressure. The shower dribbles. Fixing it means adding a pump (requires access to the loft) or upgrading the entire hot water system — both much harder once the bathroom is finished.
How to avoid: Test water pressure before selecting any shower hardware. We do this at every initial consultation.
5. Choosing Form Over Function
Cost to fix: £1,000 – £5,000 (replacement fixtures + labour)
That beautiful vessel basin that splashes water everywhere. The frameless glass shower door that leaks at the hinges. The integrated basin/countertop with no overflow that floods when left running. Design should serve function, not override it.
How to avoid: Ask your installer whether your fixture choices are practical. Experienced bathroom fitters know which products look great in showrooms but cause problems in daily use.
6. Hiring the Wrong Fitter
Cost to fix: £5,000 – £15,000+ (complete redo)
The most expensive mistake of all. An unqualified or inexperienced fitter can produce tiling with uneven grout lines, plumbing that leaks behind walls, waterproofing that fails, and electrical work that doesn't meet regulations. The only fix is often to strip everything and start again.
How to avoid: Check references, view previous work in person, verify insurance and qualifications, and be wary of quotes significantly below market rate.
We pride ourselves on getting every bathroom right the first time. Get in touch to discuss your renovation.
Frequently Asked Questions
What's the most common renovation mistake you see?
Inadequate waterproofing, by far. We regularly fix bathrooms where the previous installer didn't tank properly — water damage behind tiles is invisible until it's severe.
Should I buy my own fixtures or let the fitter source them?
Your fitter likely has trade accounts with discounts. Even if you choose the products yourself, letting your fitter order them can save 10-30% and ensures the right items are delivered on time.
How can I check if a bathroom fitter is qualified?
Ask for references and photos of recent completed work. Check for public liability insurance. For electrical work, verify Part P registration. Check Checkatrade or Trustpilot reviews.
2026 Update
Refreshed for 2026. Costs, timelines and lead times change year to year, so treat figures as a guide and ask for a current fixed-price quote. Get yours here.
